Repotting a plant is one of the most exciting parts of taking care of plants: you get to see their root development, choose a pot you think best suits the plant, and give your plant a potting mix that will promote growth while fitting your personal plant care habits. Making the right choice for a plant is not always easy, and ultimately comes down to your decision on how to best care for it. Mistakes in potting your plant can hinder its growth, and in the worst case, can contribute to the loss of a plant. I’m presenting the basics about potting mix and repotting in a series of four posts on different topics. Though there’s so much to be said about the media you could choose for a plant’s growth, the best way to learn is by practicing what you know and trying new methods that sound interesting to you. There is no single right way to pot your plants, and though there are some guidelines to help them thrive, many choices come down to your personality, the materials you have access to, and the plants you choose to grow.
Important terms: What’s the difference?
Soil. The natural medium for growth for many terrestrial plants: composed of layers of eroded rocks and minerals and organic matter, inhabited by a community of plants, animals, and microorganisms which contribute to the production and decomposition of the organic and inorganic parts of the soil. Soil types are often measured by their sand/silt/clay ratio, which tell us about the size of the particles and the drainage of the soil.
Potting mix. An artificial medium for growth for plants, usually a blend of organic and inorganic materials which resembles soil, that helps them grow in containers.
Substrate. A material that a living organism grows in (or on). Includes natural and artificial media for plant growth. Also includes the material that aquatic plants grow in inside of a tank.
Medium. The substance in which an organism grows, often with nutrients for growth.
Potting mixes (or potting media) for plants are not equivalent to the soil you’d find in your garden. Natural soils contain layers of decaying organic materials and rocks and minerals which have been broken up over years of erosion. Potting mixes are generally pretty sterile: though they contain lots of organic materials, they lack the complex ecosystems present in natural soils. You might wonder why potting houseplants with soil from your garden is generally not advised, and that is partially because of the unknown organisms you could be bringing into your space. Garden soil is usually more dense than potting mix, and dries out too slowly for plants growing indoors. This is fine for your garden plants because the light reaching garden beds is much stronger than that that sunlight that comes through windows, and so the substrate needs to be water-retentive to keep plants healthy. There are also organisms outdoors, like insects, earthworms, and small animals which aerate the soil by moving around in it. This is not the case indoors, so we need to create an an environment with sufficient air flow in a potting mix.
Potting mix has a lot more drainage and organic material than garden soil. Using potting mix specific for indoor plants helps you control moisture better for your plant and limits the kinds of organisms that make their home inside the potted plant. Healthy soils contain entire communities of organisms, many which don’t negatively affect us, which help control the populations of harmful microbes, usually by competition or exclusion. Potting mix lacks this complex community of microbes. For this reason, it’s uncommon to find mold or fungus growing there; potting mix provides all the nutrients and moisture needed by these organisms to grow and reproduce. Improper watering can contribute to this imbalance in the microbial community of your potted plant, which is why potting mix that is too moist can lead to root rot of your plant, caused by pathogenic (disease causing) microorganisms proliferating inside wet potting mix. White fuzzy mold and mushrooms growing in plants is not necessarily bad for them, though the spores can be harmful to us. In soil, plants and fungi work together to get nutrients, and there are many complex positive, neutral, or negative interactions between the organisms living together. It’s worth mentioning that humans also have complex communities of microbes which live in our gut and help us digest food, as well as stimulating healthy immune responses on our part. Without microbes, we lose out. A plant probiotic can help manage the microbial community composition inside of our plants.
Examples of different potting mixes in my home: brown ground sphagnum moss, peat, coco coir; white perlite; sparkly vermiculite; chunky orchid bark and coco chips; dark gray horticultural charcoal.
What makes a good potting mix?
A good potting mix should benefit you and your plant and promote growth.
Water retention & drainage. A good potting mix will retain water long enough to hydrate your plants, but dry out fast enough to prevent root rot and mold on the soil. I talk more about frequency of watering in my post Houseplants 101: Water, which depends on factors like light, temperature, and humidity. You can see more detailed information about potting mix components and their effects on drainage in that post.
Stability & air flow. Your plant should stand up straight in its potting mix after it’s been watered and when it’s beginning to dry out. You can add a stake to support a taller plant, or bury it a little deeper during the next time you pot it. Particle size also plays a role in plant stability: large particle size media, like orchid bark, are best for plants with strongly gripping roots, but may not support other plants so well. These media also provide really good aeration to the plant roots. Small particle size media, like ground sphagnum or sand, pack together nicely to support a plant, however the air flow to the roots is lower. A great potting mix blend balances air flow and stability for your plants. This exact balance depends on the plant, which is why manufacturers make different mixes for orchids and bromeliads, cacti and succulents, and tropical plants.
Nutrition. In nature, soil contains nutrients that help plants grow. There are also microorganisms and fungi which have complex mutualistic relationships with plants, providing nutrients in exchange for sugars that plants produce. The base components for many potting mixes (such as perlite, coco husk, pumice, etc.) don’t provide any nutrition to the plant. However, adding compost or worm castings (ie. worm poop) can help supplement your plant’s nutrition. Using a fertilizer regularly can also help feed your plant, if the potting medium you use is low in nutrients.
Longevity. No potting mix will last forever. The slow processes of erosion from watering and plant root growth will deteriorate all potting media over time. Most of the time, potting media rich in organic materials need to be refreshed every few years, and media rich in inorganic materials will last longer. One way to extend the shelf life of your potting mix is to keep it on the dry side, which will hinder the growth of microbes which could be breaking down the organic material. Using a high quality potting mix can help you reduce waste down the line, and some materials (like pumice, clay pebbles) can be reused for other plants or projects.
Potting mix and soil are not the same, which is an important fact to consider when caring for your plants: the conditions of a potted plant living in your home are much different from the natural conditions of most plants. Your potting mix can work with you to provide the most favorable conditions for plant growth, though the exact medium you use may be very different from other gardeners. I am looking forward to discuss potting mix and repotting plants in some upcoming posts. Thanks for reading!
Want to learn more about plant care basics?
Here are some other topics that might be useful:
More information about soil: